The “snake detection theory” holds that snakes played a significant role in the evolution of humans and other primates. They molded our brains, shaped our visual systems, and helped us survive. Now there is new evidence to back up this unusual theory, which explains both our agile minds and our uncanny ability to sense the presence of snakes.

Predators and Brain Evolution

The snake detection theory is the brainchild of Lynne Isbell, an anthropologist and behavioral ecologist at the University of California, Davis. She came up with the theory after spending years trying to explain a peculiar encounter she had with a snake in 1992. On that fateful day, Isbell was running through a glade in Kenya when she spotted a cobra, causing her to freeze in her tracks before her conscious brain had a chance to recognize what she saw. She later surmised that her potentially life-saving reaction was the result of millions of years of evolution.

According to her theory, which she first proposed in a 2006 article in the Journal of Human Evolutionary and later expanded upon in her 2009 book, The Fruit, the Tree, and the Serpent, snakes provided a selective pressure that allowed us to develop our advanced visual system and enlarged brains. But why snakes?

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Avian predators, on the other hand, evolved a good 20 million years after the first mammal-eating snakes, Isbell said, adding that other terrestrial carnivores, such as bears, big cats and wolves, were the last to arrive on the scene.

Given that snakes had a long history of preying upon early mammals, including proto-primates, the animals had to develop ways to avoid being eaten by their slithering enemies. For some animals, this may have meant becoming faster and more agile, or being able to detect snakes by their scent. Primates, on the other hand, developed a better visual system. “Primates are the lineage that happened to be in living conditions that were conducive to visual expansion,” Isbell said. “In order for vision to work, you need sunlight, and unlike other mammals, primates didn’t burrow.”

You can see large carnivores from afar, but the same is not always true for snakes: To pick out camouflaged snakes, you need great close-range vision. So to spot snakes better, primates evolved to have color vision and forward-facing eyes, which improves depth perception and allows 3D vision. They also evolved to have the best visual acuity among mammals, Isbell said. These visual features, which required the enlargement of some parts of the brain, were co-opted for other purposes, such as social interactions and reaching and grasping for objects.

Interestingly, the evolutionary interaction between snakes and mammals was not a one-way street, according to the snake detection theory. As mammals became better able to evade snakes — which till this point relied on squeezing their prey to death — the reptiles needed a new, easier way to kill. So they evolved venom. In response, primates evolved even better vision. Indeed, primates that live in areas without venomous snakes, such as on Madagascar, have poorer vision than other primates.

More recently, Öhman and his colleagues compared how quickly people detected snakes and spiders. Based on Isbell’s snake detection theory, they predicted that participants would detect snakes more rapidly than spiders because the arachnids were historically less of a threat to primates — and this is exactly what they found. They also discovered that snakes are more distracting than spiders, and concluded that “attending to snakes might constitute an evolutionary adaptation.”

Last year, another researcher tested how distracting snakes are during a visual perception task. She had 60 participants try to detect a bird among pictures of fruits, and every now and then she’d replace a fruit with a picture of a snake, spider or mushroom. When the “perceptual load” was low — that is, the number of fruits on the screen was low — the participants were distracted in their task more or less equally by the unexpected objects. But when the perceptual load was high, they became more distracted by the snake than by the spider or mushroom (they were equally distracted by these two stimuli). The scientist suggested that the brain preferentially processes snake stimuli, even when your attention is being demanded by other things.

In 2008, researchers found that even preschool children have enhanced snake-detecting abilities. In some trials, they embedded a single image of a snake among a bunch of nonthreatening distractor photographs of flowers, frogs or caterpillars, and in other trials they did the opposite (for example, a caterpillar among other images). The children were able to detect the snake image faster than the nonthreatening images, despite the visual similarities of snakes and caterpillars.

Recent research in the Journal of Experimental Child Psychology built upon this 2008 work. Instead of comparing snakes with caterpillars, the scientists used lizards, which are “much more similar in body morphology and tessellated scale patterns that characterizes them as reptiles.” They found that, again, the preschoolers (and adults) were better at detecting snakes than lizards. They also found that the participants could detect lions — another “historically dangerous felid predator” — faster than similarly colored antelopes. However, the study didn’t test if people are better able to detect snakes than lions (or other primate predators, for that matter).

Snake-Recognition Neurons?

Though these studies appear to support the snake detection theory, neurobiology has remained mum on the subject — until now. In a new study, just published in the journal PNAS, Isbell teamed up with neuroscientists in Brazil and Japan to see if there are structures or neurons in the brain that help primates rapidly and automatically detect snakes.

The team focused on the pulvinar, a group of neurons situated in the thalamus. “Earlier research had showed that the pulvinar, in general, is a visual structure that is mostly involved with eye movements or orienting the eyes to relevant objects in the environment,” Isbell said. In effect, the pulvinar helps mammals rapidly detect potential threats, and it’s especially important for primates. “It has expanded considerably in primates and the medial and dorsolateral parts of the pulvinar are unique to primates.”

The researchers implanted electrodes into the brains of two Japanese macaques (Macaca fuscata), which were born in captivity and have never encountered snakes before. They then measured the neuronal responses of the medial and dorsolateral pulvinar as they showed the primates four types of images: Snakes, both coiled and uncoiled; macaque faces that were either angry or neutral; macaque hands; and geometric shapes, including circles, squares and crosses.

Of the 91 neurons tested, 40.6 percent were “snake-best” — they were more active during the snake images than the other images. Comparatively, 28.6 percent were face-best neurons, 18.7 were hand-best and 12.1 percent were shape-best. It’s not too surprising that many neurons responded to the macaque faces because faces are extremely relevant to the primates, and they need to know when group members are angry (and are thus a threat), Isbell explained.

Importantly the snake images elicited the strongest and fastest responses from the neurons. For instance, the neurons responding to the snakes jumped into action about 25 milliseconds faster than the shape-best neurons and 15 milliseconds faster than the neurons responding to the angry faces.

As a follow-up experiment, the researchers also showed the macaques completely scrambled versions of the images, and images that were blurry. The neurons didn’t really respond to the scrambled images, but they did spike at the sight of the coarse images, which makes sense. It takes time and focus for the conscious mind to process visual details and understand what you are looking at, but the pulvinar needs to react quickly, before that information comes in.

“This is the first neuroscientific evidence to support the idea that snakes have been a very important selective pressure in the evolutionary history of primates,” Isbell said. Future neurobiological work will need to test neurons in other areas of the brain, as well as other primates, such as those that didn’t evolve under the pressure of venomous snakes.

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“In the future, it would also be nice to see what else gets picked up out of the animal's natural environment.” For example, how does the brain respond to raptors or leopards, compared with snakes? And if the brain responds more quickly to another predator, does that disprove the snake detection theory?